Automatic volume control circuit



NQV. 12, 1940. E FOSTER 2,221,086

AUTOMATIC VOLUME CONTROL CIRCUIT Filed Sept. 1'7, 1938 INPUT RES/STANCE /W -/0 G) BIAS INVENTOR.

DU LEY E. FSTER A TTORNEY.-

Patented Nov. 12, 1940 UNITED STATES AUTOMATIC VOLUME CONTROL CIRCUIT Dudley E. Foster, South Orange, N. J., assignor to Radio Corporation of America., a `corporation of Delaware Application september 17, 193s, serial No. 230,379

12 Claims.

My present invention relates to automatic volume control circuits for radio receivers adapted to operate in the ultra high frequency ranges, and more particularly to an improved volume control circuit employed in a system for receiving signals of the order of to 100 megacycles.

One of the main objects of my present invention is to provide a method of` controlling the transmission of signals through a receiving system of the ultra high frequency type,` the method including the utilization of an electronic device, across at least one tuned circuit of the system, which is capable of producing varying negative and positive resistance effects in response to signal carrier amplitude variation.

Another important object of the invention is to improve automatic volume control circuits for tunable ultra high frequency amplifiers; the control circuit comprising a tube having normally a negative input resistance which increases in value with control bias increase, finally assuming high positive resistance values whichdecrease with further increase of control bias.

Another object of my invention is to utilize, in an automatic volume control (A. V. C.) circuit, a tube so constructed that it has a negative input resistance; the input resistance being connected in shunt to a tuned circuit of a signal transmission system and being varied in magnitude by a control bias derived from the signals.

Still other objects of my invention are generally to improve automatic gain control circuits for ultra high frequency systems, and more especially to provide volume control circuits which are reliable, eflicient and readily assembled in a receiving system.

The novel features which I believe to `be characteristic of my invention are set forth in particularity in the appended claims; the invention 40 itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organiza-4 tion whereby my invention may be carried into effect.

In the drawing: Fig, l is a circuit diagram of a receiving system embodying the invention, -0 Fig. 2 graphically shows the operation of the control network.

Referring now to Fig. l, there is illustrated in purely schematic manner `a signal transmission system, such as a radio receiverwhich may 55 be of the superheterodyne or tuned radio frequency types. Since the former type of receiver is commonly employed for signal reception, let it be assumed that the signal collector A feeds collected signals to the tunable input circuit l of the first radio frequency amplifier of a.

superheterodyne receiverfThe input .circuit comprises coilr2 `and Variable condenser 3 providing a parallel resonant network tunable over the desired modulated signal ,carrier frequency range. Since the presentinvention is of especial utility in` the reception of signals ofthe order of 10 to 100 Mc., itV will be assumed that the condenser 3 is adjustable'to tune circuit l over a range of 20 to 50 Mc., for example. The receiver .can be of the` type `used for television reception. The `tube 4 can be an,185l type amplier; such a tube is a highkmutual conductance pent'ode and functions well for ultra high fre--` quency amplification.

The cathode circuit of tube 4 includes the usual condenser-resistor bias network; `the input .circuit l is connected between the input electrodes of tube ll. The plate and screen electrodes of the amplifier 4 may be maintained at positive potentials by the` usual voltage supply source. The numeral 6 denotes the conventional networks following the R..`F.ampl er of a superheterodyne receiver. Y the input circuit of the rst detector, or it may be connected to a seoondlR. F. amplifier. Those skilled in the art are fully aware'thatthe networks in circuit E will comprise a converter of the combined, or separate, local oscillator-first detector type; one or more I. F. ampliers;` a

second detector; one-or more modulation ampli-v fiers and a finalreproducer. V'Ihe A. V. C. bias may be secured in any well known manner. For example, the D.y C. voltage component of the second detector output may be employed. Again,

a separate diode rectifier, fed with amplied I. F.

signals, can provide the necessary A. V. C. bias.

Regardless of the source of A. V. C. bias, numeral 8 designates thelead connected to tube 9 for performing the control function. Tube 9 is of the 6K8 type, and `comprises a cathode Il),

a plate Il, a second plate l2, a grid i3 disposed between plate Il and thev cathode, and grids I4, I 5, I6 and il arranged'in the same successive order Ibetween cathode lti and plate l2. Grids I3 and lli are vconnected together, within the tube, as are grids l5 and ll. vBoth plates Il and l2 are maintained at a positive potential; the resistor-.condenser bias network 18, in the grounded cathode lead, maintains` the grids l 3l 4 at a normal negative bias. The A. V. C. lead 8, through a proper filter resistor i9, is connected to grids i3, lll and I6. l Grids l5 and il are positively biased, and hence surround grid I6 with a positive field.'

Condenser 2B connects grid I6 to the high potential side of tuned circuit I;` the input resistance R of tube 9 is shown by the dotted line representation of a resistorlin shuntto input circuit l. The resistor R has beenindicated as being capableofassuming negative or positive re- Tunable circuit 'l may `be 70 triode.

sistance values; the polarity will depend on the magnitude of A. V. C. bias applied to grid I6. In explaining the action of the lcontrol circuit it will be understood that grids I S-I i may, if desired-be connected to the ground end of bias network I 8. Further, the A. V. C. bias can be applied solely to the grids I3-I4 if desired. In

general, it can be stated that the sign and magy nitude of the input resistance R of tube S will vary in response to the change in magnitude of the A. V. C. bias; the variation will follow along the lines graphically shown in Fig. 2. From the latter it will be seen that as the A. V. C bias increases from zero to volts, by way of illustration, the negative input resistance R rises from a low value to innity and then assumes a vpositive sign and decreases to a low value.

This variation of sign and value of R is in .the correct sense for A. V. C. action. For example, when the` signal carrier amplitude is small, as during weak or distant signal reception, it is desired to have the tuned circuit I as free of damping as possible. To secure this result the shunt resistance R should be of maximum value,

`5 since the equivalent series value ofR will then be a minimum and its loading effect will be small. From Fig.2 it will be observed that for small A. V. C. bias Values, the magnitude of -R is low; hence there will be, in effect, a high value of -R in series in circuit thereby causing the circuit to be free of damping.

As the carrier amplitude increases however,

the shunt-R Value increases with a concurrent i increase of the damping of circuit I. When the sumes a positive value, the damping effect of the shunt positive R. will still be below maximum.Y

The latter is secured upon the A. V. C. bias assuming a value of 20 volts, when the shunt R 40 is small, and, therefore,the equivalent series resistanceincirouit I isa maximum. Accordingly, it will be seen. that as. the carrier` amplitude increases, the A. V. C. bias increases` in value and causes a gradual increasing loading of the cir- 45 cuit I. The variation will be suchas to maintain the carrier amplitude at theseconddetector input circuit vuniform over a Wide range of carrier amplitude .variation at collector A.

There will now be explained the nature of the 50 electronic reactions in tube 9 which permit the input resistance thereof tov vary inthe manner depicted in Fig. 2. The tube 9 acts in the manner of a triode-hexode; cathode I0, grid I3 and plate II provide a triode, Whereas cathode I0,

55 grids I4, I5, I6, II and plate l2 provide ahexode.

It can be stated that negative input resistance in the tube results from a deficiency of electrons in the space between grid I6 and screen grids I5.-I'I.

The space current between grids i4 and I5 is relatively independent of the bias on grid I6 due l to the interposed screen I5. That is to say, the sum of the currents in plate I2 and the screens I5 and I7 is relatively constant. The electrons 65, passing through grid I5 will be subject to the influence of grid I6 and` at some high negative bias on grid I6fWill be completely prevented from` passing through this grid thereby resulting in current cut-off of plate I2 as inthe ordinary However, as the negative bias on grid IIl is` decreased from some high value, plate I2 current will not decrease, linearly because the supply of electrons passing through grid I5 is limited. This limitation is brought about as fol- 75` lows: The potential of gridld exertsk the'primary A. V. C. bias passes into Vthe region where R as' influence in determining how many electrons shall be available for the remainder of the hexode section. The grid I5, by virtue of its potential and its position, will draw some value of current, Which current will be relatively independent of the potential of grid I6; the remainder of the heXode section current passing to plate I2.

Therefore, by a proper choice of potentials for grids lli and i5 there will be a scarcity of electrons onv the grid I6 side of grid I5 for some range of low values of negative bias on grid I6. Decrease in charge in the I E--IG space will cause electrons to flow into the grid I6, or current to iiow out. Thus, an increase in potential of grid I5 is accompanied by a change of opposite-sense in the current of that electrode, which condition is the criterion for negative input resistance. The net input resistance of the tubewill depend in sign and in magnitude upon the relative importance of the two effects operating simultaneously within the space between grids I5 and I6. The frequency also affects the input resistance, because the proportionatemagm'tude of the in-phase, or resistance, component depends upon the finite electron transit time, and it is for that reason that this negative resistance effect occurs only at relatively high frequencies.

The average potential of grid I6 will effect the distribution of electrons within the space 5-l6, and the average potential of grid i i will determine the number `of electrons passing through screen I5. Therefore, the bias on grids I and I6 will both affect the magnitude of input resistance R, When the tube is operated with a high value of negative bias on grid I6, the current of plate I2 will decrease and the screen grid current Will rise. These effects will be accompanied by the building up of a space charge in the space between grids I5 and It. Under these conditions the tube will simulate a triode Whose plate current is space charge limited insofar as consideration of the input resistance is concerned. For high values of negative bias on grid I the tube 9 behaves as a triode; that is to say, the input resistance is positive. The input resistance R, as it goes from negative to positive, passes through infinity, and so as bias is increased the positive resistance must decrease from infinity, as long as the tube is operating under electron deficiency condition. When the bias becomes suiciently negative this `condition is departed from and the tube acts like ordinary tubes where input resistance increases with more negative bias; however, that effect is small compared to the variation from low negative resistance through infinity and thence to decreasing positive resistance values. Of course, in place of tube 9 any other tube can be used that has an input resistance which is capable of varying in the manner illustrated in Fig. 2. Again, the control action may be applied to any other tuned circuit of the system. It can be shown that the sign and magnitude of R vary much the same as shown in Fig. 2, when the bias on grids IS-Ii is varied. y

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set for in the appended claims.

' put electrodes arranged to provide a negative in- What I claim is:

1. A method of operating a wave transmission system of the type comprising at least one tuned circuit feeding a wave transmission tube, which includes producing a minimum. negative resistance effect across the tuned circuit for waves of minimum amplitude, deriving a control voltage from the waves which varies directly in magnitude in response to Wave amplitude increase, increasing the magnitude of said effect in response to control voltage magnitude increase, and converting said effect into a positive resistance eiect of decreasing magnitude in response to further increase of the control voltage magnitude beyond a predetermined value.

2. A method of 'operating a wave transmission system of the type comprising at least one tuned circuit feeding a wave transmission tube,` which includes producing a negative resistance effect across the tuned circuit, deriving a` control voltage from the waves which varies in magnitude in response to wave amplitude changes, varying the magnitude of said eiTect in response to control voltage magnitude variation, and converting said elTect into a positive resistance effect of opposite magnitude variation in response to variation of the control voltage magnitude beyond a predetermined point.

3. A method of operating a wave transmission system of the type comprising at least one tuned circuit feeding a wave transmission tube, which includes producing a negative resistance effectacross the tuned circuit, deriving a control voltage from the waves which varies in magnitude in response to wave amplitude changes in a sense such that the control voltage is increasingly negative as the wave amplitude increases, varying the magnitude of said effect in response to control voltage magnitude variation, and converting said eiect to one of opposite sign in response to an increase in the control voltage magnitude beyond a predetermined point.

4. In a receiver including at least one tuned signal circuit, the method which includes producing a resistive effect across the circuit, and varying the sign and magnitude of the eiiect across said one circuit in a predetermined order in response to signal amplitude variation.`

5. In combination with a receiver having at least one tuned signal circuit, an electronic device providing a resistance across the circuit, said device being constructed to impart to said resistance a variable sign which depends on the gain of the device, and means, responsive to signal amplitude variations, for adjusting said device gain thereby to vary the sign of the resistance acrossV said one signal circuit.

6. In an amplifier having a tuned input circuit, an electron discharge tube having input and output electrodes arrangedto provide anegative input resistance between the input electrodes at 10W negative bias values, means connecting said input electrodes across the input circuit thereby to connect said resistance across the latter, means responsive to sign-al amplitude increase for increasing the bias on said tube thereby to increase the magnitude of said resistance up to a predetermined value of bias and thereafter to convert the sign of the resistance to be positive.

7. In an amplifier having a tuned input circuit, an electron discharge tube having input and output resistance between the input electrodes at low negative bias values, means connecting said input electrodes across the input circuit thereby to conf nect said resistance across the latter, means responsive to signal amplitude increase for increasing the bias on said tube thereby to increase the magnitude of said resistance up to a predetermined bias value and thereafter to convert the sign of the resistance to be positive, said tube comprising a cathode, a positive plate, a negative grid, a positive eld surrounding the grid, and a second negative grid between the cathode and i positive eld, said resistance being provided between the cathode and rst negative grid.

8. In combination with a receiver having `a transmission tube whose input circuit is tuned to a frequency of the order of 10 to 100 megacycles, a tube having its input resistance connected across said input circuit to control the 'gain of said receiver, said control tube beingconstructed to provide an electron deciency between a pair of electrodes thereof whereby said input resistance may be controlled in sign, and means responsive to signal amplitude variation for controlling said resistance sign. ,y

9. In a signal reception system including at least one tunedl signalcircuit, the method of regulating signal transmission therethrough which includes producing a resistive effect across the circuit, and varying the sign of the eiect across said circuit from negative to positive in response to signal amplitude increase.

10. In a signal reception system including at least one tuned signal circuit, the method of regulating signal transmission therethrough which includes producing a resistive eiect across the circuit, varying the vsign of the eiect across said circuit from negative to positive in response to sign-al amplitude increase, and varying the magnitudes of said negative and positive effects in opposite senses.

11. In a signal reception system including at least onetuned signal circuit, a method of adjusting the damping ofthe signal circuit which includes introducing resistance of a desired sign` into said signal circuit inresponse to the reception of signals of a predetermined amplitude, varying the value of said resistance in response to variation of said signal amplitude, and introducing resistance of opposite sign into said signal circuit in response to a substantial shift of the` signal amplitude from said predetermined amplitude. C

12. In a signal reception system including at least one tuned signal circuit, a method of ad'- justing the damping of the signal circuit which includes introducing` resistance of a desired sign into said signal circuit in response to the reception of signals of a predetermined amplitude, varying the value of said resistance in response to variation of said signal amplitude, introducing resistance of opposite sign into said signal circuit in response to a substantial shift of the signal amplitude from said predetermined amplitude, and varying the value of the opposite sign resistance in a sense opposed tosaid first variation in response to further change in said signal ampiitude.

DUDLEY E. FOSTER. 

